http: add docs/warning on incorrect use of NewChunkedWriter
http: add docs/warning on incorrect use of NewChunkedWriter

R=golang-dev, adg, r
CC=golang-dev
http://codereview.appspot.com/4536075

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang